Hybrid vehicles may be driven by a primary engine, which may be a combustion engine, and a secondary engine, which may be an electrical machine. The electrical machine is equipped with at least one energy storage device, such as an electro-chemical energy storage device, for storage of electric power and control equipment to control the flow of electric power between the energy storage device and the electrical machine. The electrical machine may thus alternately operate as a motor and as a generator, depending on the vehicle's operating mode. When the vehicle is braked, the electrical machine generates electric power, which is stored in the energy storage device. This is usually referred to as regenerative braking, which entails that the vehicle is decelerated with the help of the electrical machine and the combustion engine. The stored electric power is used later for operation of the vehicle.
A gearbox in a hybrid vehicle may comprise a planetary gear. The planetary gearbox usually comprises three components, which are rotatably arranged in relation to each other, namely a sun wheel, a planetary wheel carrier and an internal ring gear. With knowledge about the number of cogs in the sun wheel and the internal ring gear, the mutual speeds of the three components may be determined during operation. One of the components of the planetary gear may be connected with an output shaft in a combustion engine. This component of the planetary gear thus rotates with a rotational speed corresponding to the rotational speed of the output shaft in the combustion engine. A second component in the planetary gear may be connected with an input shaft to a transmission device. This component of the planetary gear thus rotates with the same rotational speed as the input shaft to the transmission device. A third component in the planetary gear is used to achieve hybrid operation, connected with a rotor in an electrical machine. This component in the planetary gear thus rotates with the same rotational speed as the rotor of the electrical machine, if they are directly connected with each other. Alternatively, the electrical machine may be connected with the third component of the planetary gear via a transmission that has a gearing. In this case, the electrical machine and the third component in the planetary gear may rotate with different rotational speeds. The engine speed and/or the torque of the electrical machine may be controlled steplessly. During operating times when the input shaft to the transmission device must be provided with a desired rotational speed and/or torque, a control device having knowledge about the engine speed of the combustion engine calculates the rotational speed with which the third component must be operated, in order for the input shaft to the gearbox to obtain the desired rotational speed. A control device activates the electrical machine, so that it provides the third component with the calculated engine speed and thus the input shaft to the transmission device with the desired rotational speed.
By connecting the combustion engine's output shaft, the electrical machine's rotor and the transmission device's input shaft with a planetary gear, the conventional clutch mechanism may be avoided. At acceleration of the vehicle, an increased torque must be delivered from the combustion engine and the electrical machine to the transmission device, and further to the vehicle's driving wheels. Since both the combustion engine and the electrical machine are connected with the planetary gear, the largest possible torque delivered by the combustion engine and the electrical machine will be limited by one of these drive units; i.e. the one whose maximum torque is lower than the second drive unit's maximum torque, having regard to the gearing between them. In case the electrical machine's highest torque is lower than the combustion engine's highest torque, having regard to the gearing between them, the electrical machine will not be able to generate a sufficiently large reaction torque to the planetary gear, entailing that the combustion engine may not transfer its highest torque to the transmission device and further to the vehicle's driving wheels. Thus, the highest torque that may be transferred to the transmission device is limited by the electrical machine's strength. This is also apparent from the so-called planet equation.
Using a conventional clutch, which disconnects the gearbox's input shaft from the combustion engine during shifting processes in the gearbox, entails disadvantages, such as heating of the clutch's discs, resulting in wear of the clutch discs and an increased fuel consumption. A conventional clutch mechanism is also relatively heavy and costly. It also occupies a relatively large space in the vehicle.
In a vehicle, the space available for the drive arrangement is often limited. If the drive arrangement comprises several components, such as a combustion engine, an electrical machine, a gearbox and a planetary gear, the construction must be compact. If there are additional components, such as a regenerative braking device, the requirements that the component parts must have a compact construction are even more stringent. At the same time, the component parts in the drive arrangement must be designed with dimensions that are able to absorb the required forces and torque.
For some types of vehicles, especially heavy goods vehicles and buses, a large number of gear steps is required. Thus, the number of component parts in the gearbox increases, which must also be dimensioned to be able to absorb large forces and torque arising in such heavy goods vehicles. This results in an increase of the size and weight of the gearbox.
There are also requirements for high reliability and high dependability of the components comprised in the drive device. In case the gearbox comprises multi-plate clutches, a wear arises, which impacts the reliability and life of the gearbox.
At regenerative braking, kinetic energy is converted into electric power, which is stored in an energy storage device, such as accumulators. One factor impacting on the life of the energy storage device is the number of cycles in which the energy storage device provides and extracts power to and from the electric machines. The more cycles, the shorter the life of the energy storage device.
Under some operating conditions, it is desirable to shut off the combustion engine, with the objective of saving fuel and to avoid cooling down of the combustion engine's exhaust treatment system. When a torque injection is required in the hybrid powertrain, or when the energy storage device must be charged, the combustion engine must be started quickly and efficiently.
The document EP-B1-1126987 shows a gearbox with double planetary gears. Each sun wheel of the planetary gear is connected to an electrical machine, and the internal wheels of the planetary gears are connected with each other. The planetary wheel carrier in each planetary gear is connected to a number of gear pairs, so that an infinite number of gear steps is obtained. Another document, EP-B1-1280677, also shows how the planetary gears may be bridged with a gear step arranged on the combustion engine's output shaft.
The document US-A1-20050227803 shows a vehicle transmission with two electrical machines, connected to the respective sun wheels in two planetary gears. The planetary gears have a common planetary wheel carrier, which is connected to the transmission's input shaft.
The document WO2008/046185-A1 shows a hybrid transmission with two planetary gears, wherein one electrical machine is connected to one of the planetary gears and a double clutch interacts with the second planetary gear. Both planetary gears also interact with each other via a cogwheel transmission.